This invention relates in general to high voltage shutdown circuits for cathode ray tubes (CRTs) and in particular to such shutdown circuits that are utilized in systems having regulated high voltage supplies and separate deflection systems.
In projection monitors and television receivers, the light from the individual color projection CRTs is projected to a physically displaced viewing screen. In order to produce acceptable illumination, the projection CRTs are driven "hard", i.e. at large beam current levels, which requires not only high levels of high voltage but fairly "stiff" (regulated) high voltage supplies. To limit x-radiation emission from the supplies and from the CRT, the high voltage is generally kept below a safe value of about 31KV.
A high voltage shutdown circuit is provided for sensing the voltage across the high voltage transformer and for initiating trigger means when that high voltage exceeds a particular value, i.e. when the actual voltage will be in excess of the safe level. Operation of the trigger means develops a shutdown signal that is effective to disable the high voltage supply. High voltage shutdown circuits are included to guard against abnormal operating conditions as well as circuit failures that would result in excessive high voltage levels.
As mentioned, increases in CRT brightness levels require that more energy be supplied from the high voltage system. In regulated high voltage systems in particular, the peak amplitude of the voltage developed across the high voltage transformer is increased significantly in order to supply the additional energy. While the high voltage present at the CRT is a function of the maximum voltage across the high voltage transformer, the increased brightness results in significant beam current loading on the high voltage circuit. While the regulator is increasing the voltage across the high voltage transformer to enable it to supply the additional energy demanded for the high brightness display, the actual high voltage developed no longer bears the same relationship to the voltage across the transformer. In short, while the voltage across the transformer certainly rises, the rise is not necessarily indicative of a proportional increase in developed high voltage. The result is that with an increased display brightness in such a system, the voltage spread between normal operation and high voltage shutdown is effectively narrowed and premature high voltage shutdown can occur.
The circuit of the present invention corrects such premature high voltage shutdown by developing a tracking voltage that is responsive to the brightness or energy level of the CRT and using that tracking voltage to adjust the operating point of the triggering means accordingly.